The speech processor for the cochlear implant is a body worn device which receives audio signals from an ear level microphone and transmits an encoded RF signal to the implanted device via a coil located over the implant site. Thus, there is a requirement to transfer a low level audio signal (typically 1 millivolt RMS) from the microphone to the speech processor and a high level signal (approximately 10 volts RMS) back to the coil. This represents an amplitude difference of 80 dB.
The high level signal is generally a wide band burst modulated RF carrier where the bursts are encoded stimulus data. Typically, the burst modulated carrier includes frequency components in the audio frequency range. The low level signal is an audio signal with a bandwidth of interest of 100 Hz to 10 KHz. The high level RF signal, if burst width modulated, has significant frequency components within the audio spectrum. It is therefore necessary to take special precautions to minimize crosstalk between the two signals. In the past this has been achieved by the use of a custom made multicore screened cable, which provided independently shielded conductors for the microphone connections and lower capacitance conductors for the transmitter coil connections. While this achieves the necessary isolation between the two signals, it requires a special cable which is heavier than ideal and very expensive to manufacture and to terminate to connectors.
Body worn hearing aids have made use of a light weight three wire twisted cable which is both readily available able and inexpensive. Several manufacturers provide standard length cables terminated with standardized three pin IEC plugs. These cables are relatively low in capacitance, but are not screened. However, because of the abovementioned crosstalk difficulties, the audio and RF circuits used in prior art cochlear implant systems cannot utilize only three unshielded conductors.
FIG. 1, illustrates a prior art five wire system for connection of a processor 10 having an audio signal processing circuit 10A to a headset 11. An RF transmitter 12, within processor 10, operates in class-E mode, i.e. the driver transistor 13, which is connected in series with a diode 14 and a damping transistor 16, conducts for approximately fifty percent of the total cycle and switches when the voltage across it is close to zero. A transmitter coil 18 and a capacitor 20 form a tuned circuit, which is connected to transmitter 12 by conductors 22A and 22B of five wire cable 23.
A microphone 22 is connected to audio signal processing circuit 10A of processor 10 via an independently shielded three wire portion 28 of cable 23. Cable 23 includes a power conductor 29A and audio conductor 29B, shielded by a braid 29C connected to a ground conductor 29D. A D.C. bias supply is filtered by a resistor 30 and a capacitor 31 to minimize noise of power supply origin. A five wire connector 32, which is generally fairly expensive, is used to connect cable 23 to processor 10.
The waveforms associated with the circuit of FIG. 1 are illustrated in FIGS. 2A to 2D. The BURST waveform of FIG. 2A is applied to the gate of transistor 13. The NDAMP waveform of FIG. 2B (the logical opposite of a damp waveform) is applied to the gate of transistor 16.
As illustrated in FIG. 2C, during the cycle, current in coil 18 changes approximately linearly from a peak negative value to a peak positive value. During the half cycle in which transistor 13 is turned off, current cycles sinusoidaly from a peak positive value back to a peak negative value. As illustrated in FIG. 2D, the voltage waveform across the wires 22A and 22B and coil 18 is asymmetrical, swinging from -5 volts to +25 Volts around the +5 volt supply, during the on and off times, respectively, of driver transistor 13. This extreme asymmetry in coil drive waveform results in significant low frequency components which are readily coupled into the audio signal processing circuit 10A.
The mutual coupling between transmitter coil 18 and a receiver coil (not shown in FIG. 1) increases as the distance between them decreases. The load across transmitter coil 18 increases accordingly, and this results in increased power to the transmitter. Ideally, the power consumption should decrease as the coupling increases.
As illustrated in FIG. 3, the five wire system of FIG. 1, which uses the transmitter coil as the inductor in the class-E stage, exhibits characteristics opposite the ideal, whereby closer coupling reflects a lower impedance across the coil. This results in more energy being taken from the tuned circuit and consequently the transmitter draws more power.
Another disadvantage of the five wire system illustrated in FIG. 1 is that it is necessary to have a +/-5 volt power supply. Such a dual voltage power supply adds considerable cost to the system.
Prior attempts to achieve better efficiency have used stagger tuning of the transmitter and a receiver using a fixed transmitter frequency, or a self oscillating transmitter circuit which detunes as coupling coefficient increases. This latter approach is not usable with a fixed frequency transmitter which is preferred for the encoded pulsatile stimulation strategy.